Sorted by IM3

Sorted by IM3

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Sorted  by Intermodulation. Best receiver first. 

This is a measure of
how badly a close-spaced signal will trash your receiving with spurious signals
or splatter sounds, and you might blame it on the other guy thinking your
receiver is just great. Remember that even in my very quiet rural location
where I have to “beam” through the east coast to hear Europe, 80dB or
so IM3 is probably more than enough. The reason my Drakes are so good is because
they were built to duplex on the same band with my own transmitters running!!
Unless you are in a situation like that, anything over 80dB is likely wasted.

Most transmitters
are really the
source of problems.
Exceptions to
transmitters being a primary source of 
off or adjacent
channel interference occurs

1.) You use a noise blanker. Most noise blankers kill receiver IM and
blocking performance.

2.) You don’t have good narrow IF filters.

3.) You use too much gain in the receiver front end.

4.) You have a receiver with a design problem (less than 80-85dB close-spaced
IM3 DR) . 

Beyond the above, most transmitters are the real problem. Look at transmitter
IM tests on SSB and read about keyclicks. 

Poor Designs

Poor designs include:

1.) Many different Yaesu’s without noise blanker mods. NO, changing the
diodes doesn’t do a thing! Been there, done that. The hissing noise comes from a
FET following the narrow filters, the IM comes from the silly Yaesu
noise-blanker design that hangs an FET right on a wide IF line with the
potential for that FET to be at full gain even when the NB is OFF!!!

2.) Early DSP filter only radios.
Many aren’t that good. If you have a DSP radio
without a
roofing filter that matches the mode
, you
could have a poor receiver.
There are
exceptions, but they
are still not as
good as a better
narrow crystal or
mechanical filter

3.) Radios with spurious response problems. The JRC JST-245’s I’ve tested and
used all test good, but they have so many spurious synthesizer responses on 160
meters I’d never consider using one for serious weak signal work. I don’t know
how they are on other bands, but the
spurious responses
make them poor
performers on 160.

You can see how the
noise blanker FET in the
Yaesu hurts the performance.
The W8JI modified
Yaesu’s have the
noise blanker
transistor modified
to turn fully off.
The stock early R4C
uses a dual gate
MOSFET mixer that is
subject to overload,
and is one of the
worse receivers
10KHz or wider tests
are almost
meaningless. Almost anything is good at 10kHz, even some very poor
receivers.   Here’s how a few receivers stack up:

Model (in some cases serial included) MDS 2kHz  BDR 10kHz BDR 2kHz IMDR 10kHz IMDR
1 R4C heavy mod. Experimental. 

See note 5

-143 127 131 118 119 
2 R4C 17816 med mod. 

See note 4

-140 96 115 92 103 
3 TenTec Orion -129 98 100 92 92
4 FT1000D/W8JI mods -136





with W8JI NB
Mods (Inrad
roofing removed)





5 TS-870 Inrad 400Hz IF filter mod (AIP) -139










6 FT1000MK V

W8JI mods

-131 80 89 85 90
7 IC-751A -140 83.5 98 79 91
8 IC-7800 -127 80  98 78 88@5 kHz


W8JI NB mods
with Inrad
roofing filter








-137 pre1 on

-127 no preamps






FT1000MP 8N370165 -135 73.5 84 62*      75.5**

note 1

70.5*  80**  

note 1

FT1000D -134 68 85 72 81

R4C S/N 22291 stock -139 80 116 68 86 note3

IC-756 PRO -133 N/A 78 67 77

FT1000MK V stock -131 74 86 60 note1A 77 note 1A

R4C S/N 17816 stock -132 57 109 48 82 note2


Note 1: The FT1000MP has the same inherent problem as the FT1000 and FT1000D.

A noise-blanker amplifier FET is left on even when the noise blanker is turned off with
front panel NB and NB-W switches

This FET connects directly to the IF, and overloads when strong signals are
present. Unlike the FT1000 and FT1000D, the front panel noise blanker level
control affects the FT1000MP IMD performance. * is with the NB gain control fully
advanced, ** is with the NB gain control fully off. In ALL cases the NB
off-on switch is OFF!!

Note 1A: Yaesu copied the same basic NB from the FT1000D, and
so this radio also has a NB problem even when the NB is switched off. This test
was with NB gain menu set for minimum gain. The radio is worse when NB gain is
increased in the NB gain menu.   

Note 2: This is a MOSFET 2nd mixer R4C, aligned and in good shape but with no
changes. This receiver is almost useless for weak signal CW work of any kind
because of poor IM performance and filter blow-by.  

Note 3: This is a tube 2nd mixer R4C, aligned and in good shape but with no
changes. This receiver is almost useless for contesting or crowded bands because
of poor IM performance and bad filter blow-by. 

Note 4: This is the same receiver in note 2, but with the following changes:

  1. Increased screen voltage on RF amplifier to 130 volts.
  2. 6J6 first mixer with oscillator injection increased to 5 volts.
  3. Diode double-balanced 2nd mixer with +10dBm injection.
  4. 10dB of gain removed from first mixer and first IF and redistributed
    behind the narrow filter.
  5. Sherwood Engineering 600Hz roofing filter.
  6. New AGC system. After narrow filter only, no AGC on front end.

Note 5: Gutted and rebuilt with solid-state double balanced high-level
mixers. No vacuum tubes in signal path. Sherwood 600Hz roofing
filter. Experimental receiver for full-duplex operation on 160 meters. This
receiver allows me to hear weak DX while I am transmitting, as long as they are
a few kHz off frequency.